Method for sealing anodized aluminum

ABSTRACT

A COMPOSITION FOR SEALING ALUMINUM OXIDE COATING ANODICALLY FORMED ON ALUMINUM AND ALUMINUM BASE ALLOYS COMPRISES AN AQUEOUS SOLUTION CONTAINING AT LEAST ONE G./LITER OF OXYGEN ACID ION OF PHOSPHORUS AND AT LEAST 0.2 G./LITER OF ALKALINE EARTH METAL ION AND HAVING A PH OF 2.5 TO 9.0, AND A METHOD FOR SEALING THE ALUMINUM OXIDE COATING COMPRISES CONTACTING THE COATING WITH SAID COMPOSITION AT 20 TO 50*C. AND DRYING THE RESULTANT COATING.

United States Patent 3,749,596 METHOD FOR SEALING AN ODIZED ALUMINUM Chozo Yoshimura, Nara, Japan, assignor to Okuno Chemical Industry Company Limited, Higashi-ku, Osakashi, Japan No Drawing. Filed May 11, 1971, Ser. No. 142,416 Claims priority, application Japan, May 13, 1970, 45/ 41,183 Int. Cl. C231? 7/06 US. Cl. 117-127 1 Claim ABSTRACT OF THE DISCLOSURE A composition for sealing aluminum oxide coating anodically formed on aluminum and aluminum base alloys comprises an aqueous solution containing at least one g./liter of oxygen acid ion of phosphorus and at least 0.2 g./liter of alkaline earth metal ion and having a pH of 2.5 to 9.0, and a method for sealing the aluminum oxide coating comprises contacting the coatin with said composition at 20 to 50 C. and drying the resultant coating.

This invention relates to a composition for sealing aluminum oxide layers anodically formed on aluminum or aluminum base alloys.

The anodically formed aluminum oxide coatings are microporous and have very high adsorptive properties. Because of these characteristics, such coatings can be dyed in a wide variety of colors. However, these characteristics which facilitate coloration result in the drawback that when the colored anodized coating is put to use as it is, the coating is liable to staining and once the coating is stained, it is almost impossible to remove the stains. In order to eliminate this drawback, to improve corrosion resistance and to prevent discoloration of the colored anodized coating, it has generally been practiced to subject the anodized coating to a treatment for sealing the micropores therein.

The above-mentioned sealing treatment can be provided by treating the anodized coating with steam under an elevated pressure, with boiling water or with an aqueous solution of nickel acetate or an aqueous solution of cobalt acetate and nickel acetate at a temperature of higher than ing. For these reasons, sealing treatment is generally car-' ried out with boiling water or a nickel acetate bath or cobalt acetate-nickel acetate bath in recent years. However, the treatment with boiling water still permits the dye to dissolve out from the colored anodized coating, while the fact that the treatment with nickel acetate or cobalt acetate-nickel acetate bath has to be conducted at a temperature of at least 80 C. is inevitably accompanied by an increased equipment cost for heating operation when large aluminum articles are to be treated as in recent years. At any rate, all of the foregoing methods require a considerable length of time, hence ineflicient.

It has also been proposed to use dichromate, molybdic acid, water glass, metal chelate or the like, but any of these substances, unless used at a temperature of higher than 80 C., fails to achieve effective results and to reduce the treating time. The use of these substances is not of industrial advantage therefor.

Accordingly, the primary object of this invention is to provide a composition which is capable of effectively sealing an anodized coating on aluminum or aluminum ice base alloys at around room temperature within a short period of time so as to impart to the coating corrosion resistance and abrasion resistance as high as heretofor been achieved.

The above and other objects of this invention will become apparent from the following description.

The term anodized coating as used herein is intended to mean an aluminum oxide coating anodically formed on aluminum or aluminum base alloys.

The sealing composition in accordance with the present invention comprises an aqueous solution containing at least one g./liter of oxygen acid ion of phosphorus and at least 0.2 g./liter of alkaline earth metal ions and having a pH of 2.5 to 9.

In contrast with the conventional method which requires a high temperature treatment of more than C., the above-mentioned sealing composition of this invention is capable of imparting to the anodized coating higher corrosion resistance than has conventionally been provided and sufficiently high abrasion resistance when used at a treating temperature of about 20 to 50 C. for a short period of time.

The aluminum alloys used in the invention are those containing a minimum quantity of elements other than aluminum. Representatives thereof are (1) Al-Cu alloys including Aluminum Association numbers 2014, 2017, 2018, 2024, 2025, 2117, 2214, etc., (2) Al-Mn alloys including Aluminum Association numbers 3003, 3002, 3105, etc., (3) Al-Si alloys including Aluminum Association numbers 4032, 4145, 4343, 4543, etc., (4) Al-Mg-Si alloys including Aluminum Association numbers 5005, 5051, 5052, 5056, 5154, 5456, 5457, 5556, 5557, 5652, 5657, etc., (5) Al-Mg-Si alloys including Aluminum Association numbers 6003,6005, 6053, 6061, 6063, 6151, 6253, 6262, etc., (6) Al-Zn alloys including Aluminum Association numbers 7072, 7075, 7076, 7175, 7178, etc., and (7) others such as Aluminum Association numbers 8001, 8013, 8040, 8079, 8280, etc. The aluminum oxide coating anodically formed on aluminum or alloys thereof is usually 1 to 20 in thickness.

The sealing composition of this invention contains oxygen acid ion of phosphorus and alkaline earth metal ions. The source of the oxygen acid ion of phosphorus includes oxygen acids of phosphorus, such as hypophosphorous acid, phosphorous acid, orthophosphoric acid, pyrophosphoric acid, tripolyphosphoric acid, etc. and water-soluble salts thereof, and the alkaline earth metal ion source includes Water-soluble alkaline earth metal salts. The aqueous solution of an alkaline earth metal salt of oxygen acid of phosphorus includes-the both effective ions, i.e. oxygen acid ion of phosphorus and alkaline earth metal ion so that it is a preferred example of the sealing composition of this invention. Exemplary of such alkaline earth metal salts are calcium phosphate, magnesium phosphate, barium phosphate, magnesium ammonium phosphate, etc. Another example of the sealing composition of this invention is an aqueous solution containing an oxygen acid of phosphorus or water-soluble salt thereof and a water-soluble alkaline earth metal salt in combination. The water-soluble salts of oxygen acids of phosphorus other than alkaline earth metal salts includes alkali metal salts of oxygen acid of phosphorus such as sodium phosphate, potassium phosphate, sodium phosphite, sodium tripolyphosphate, sodium pyrophosphate, etc. The oxygen acids of phosphorus and water-soluble salts thereof may be used alone or in mixture. The watersoluble alkaline earth metal salt other than oxygen acid salt of phosphorus includes chlorides, carbonates, nitrates, acetates and the like of calcium, strontium, barium or magnesium. Such alkaline earth metal salts may be used alone or in combination.

It is required that the sealing composition of this invention contain at least one g./liter of oxygen acid ion of phosphorus and at least 0.2 g./liter of alkaline earth metal ions. If the amount of either one of these two kinds of ions is lower than the above, the sealing effect will not be fully achieved. As the concentration of oxygen acid ion of phosphorus increases, the sealing eifect improves but if it exceeds 20 g./liter, no improvement will be achieved in sealing eifect, hence uneconomical, although the use of larger amount as 50 g./liter will not be accompanied by any noticeable adverse eifect. The oxygen acid ion concentration may preferably be in the range of to 15 g./liter. Further the higher the concentration of alkaline earth metal ions, the greater will be the sealing effect achieved, but if it exceeds 20 g. /liter, there will not be any improvement attained in the sealing eifect and hence uneconomical, although no adverse effect will result. In general, the concentration of the alkaline earth metal ions may preferably be 2 to 7 g./liter.

It is required that the sealing composition of this invention have pH value of 2.5 to 9.0. If the pH of the composition is not higher than 2.5, poor sealing effect is obtained. Excess alkalinity with pH in excess of 9.0 may impart more improved corrosion resistance to the anodized coating upon sealing treatment but will cause the alkaline earth metal salt to precipitate and produce corrosion in the anodized coating. The preferred pH is in the range of 3 to 4, and the best results will be achieved with a sealing composition having a pH value in this range. The pH of the composition can be adjusted with a suitable acid or alkali, although this is dependent on what kinds of oxygen acid of phosphorus or salt thereof and alkaline earth metal salt are used. Examples of such acid or alkali used for the purpose of pH adjustment are hydrochloric acid, tartaric acid, nitric acid, sodium hydroxide, sodium carbonate, Rochelle salt, etc.

Further improvement in the corrosion resistance can be attained by addition of a specific water-soluble metal compound, water-soluble oxidizing agent or Water-soluble reducing agent to the composition of this invention. The metal compound includes (1) compounds of the copper group such as CuSO CuCl AgClO AgNO AuCl AuBr etc.; (2) compounds of the boron group such as B 43 K B O -5H O, AlCl Al(NO -9H 0, NH AlCl etc.; (3) compounds of the titanium group such as TiCl Ti(SO.;) etc.; (4) compounds of the carbon group such as Na SiO water glass (Na O-nSiO -nH O), SnCI H SnS '3-H O, SnSO Pb(ClO PbCrO etc.; (5) compounds of the nitrogen group such as SbCl KSbO K SbS -9H O, BiNO NaBiO etc.; (6) compounds of the chromium group such as CrCl 010 etc. These compounds are added alone or in combination to the composition of this invention in an amount of 0.1 to 8 g./liter, preferably 0.3 to 5.0 g./liter.

The present composition may be used in the same manner as with usual treatment with nickel acetate. Thus the treatment can be carried out by a simple method, for example, by immersion of an article to be treated in the present composition or by application of the composition by spraying or coating to the articles to be treated.

In any case, the treatment in accordance with this invention can be conducted at a temperature of about 20 to 50 C. without requiring any high temperature, with the resulting advantage that the operation procedure and apparatus can be substantially simplified and that large aluminum articles can be treated with great ease. Preferable temperature is in the range of 40 to 50 C. Moreover, treatment is completed within a markedly shortened time of about 5 to 15 minutes. The treatment is conducted on batch system or continuous system. For continuous operation, the sealing composition is supplied suitably so as to maintain the amounts of effective components at a constant level. Continuous operation may result in aging of the composition and variation in pH value due to reduction of acid or alkali. Acid or alkali may then be added suitably. It is also possible to use stabilizers for the respective components.

After the treatment with the present composition the article is washed with water, as required and dried.

For better understanding of the invention examples are given below, in which percentages are all by 'weight.

EXAMPLE 1 The following composition was prepared:

Sodium tripolyphosphate5 g./l. (phosphate ion cone.

Calcium chloride-3 g./l. (calcium ion conc. 0.82 g./l.)

Tartaric acid0.2 g./l.

WaterAmount necessary for making one liter solution Aluminum plate of alloy prescribed in JIS-AIPI and containing more than 99.5% of aluminum, less than 0.5% of silicon and less than 0.05% of copper was anodized in 10% sulfuric acid at 20:1" C. using a current density of one A./d-m. for 50 minutes to form an anodized coating.

The plate was then immersed in the composition of Example 1 at 45 C. for 10 minutes for sealing. The sealed coating was subjected to the following testsnThe results thereof are shown in Table 1 below.

(1) Corrosion resistance test In accordance with 118 H 8601-1968, a test piece was placed in the device heated to 35:1 C. and 10% caustic soda was applied dropwise to the test piece. Upon lapse of a predetermined length of time, the test piece was washed with water and dried. Electroconduction was measured by a tester and the time required for removal of the coating was determined. The result was given in terms of second.

(2) Abrasion resistance test (3) Spot dyeing test Two drops of 1 g. of Anthraquinone Violet in 50 ml. of solution were applied to a test piece separately at room temperature, and the test piece was left to stand for 5 minutes and then washed with running Water. The test piece was thereafter rubbed with cotton soaked with a cleansing agent for 15 seconds and then washed with water. Without subbing the test piece, it was dried with filter paper and checked for the formation of a spot. The results are represented by:

A: Spotless B: Faint spot C: Clear spot (4) Finger mark test A solution mixture comprising 7 g. of sodium chloride, 1 g. of urea, 4 g. of 85% lactic acid, 500 cc. of methanol and 500 cc. of water was fully applied to sealed aluminum surface with a piece of gauze soaked with the solution mixture, and the liquid was wiped off the surface with dry gauze and checked for the presence of trace of the liquid. The results are represented by:

A: Free of any trace B: Slight trace C: Fairly clear trace D: Very clear trace Comparative samples prepared by the following methods were also subjected to the same tests and the results thereof are shown in Table 1.

Comparison 1 The anodized coating the same as in Example 1 was sealed at 45 C. for minutes using the following composition:

Calcium chlorideS g./l. (calcium ion conc. 1.8 g./l.) Tartaric acid-0.3 g./l. WaterAmount necessary for making one liter solution pH3.2

Comparion 3 The anodized coating the same as in Example 1 was sealed in boiled water maintained at 98 to 100 C. for 30 minutes.

Comparison 4 The anodized coating the same as in Example 1 was sealed in the following composition at 90 C. for 30 minutes.

Nickle acetate20 g./l.

WaterAmount necessary for making one liter solution pH5.5

TABLE 1 Corrosion Abrasion Finger resistance resistance Spot mark Sealing method (see) (see) dyeing test Example 1 60 1,200 B A Comparison 1-.. 20 1, 600 C C 3 Comparison 2 20 1,600 O D 4 Comparison 3... 55 1, 400 B A 5 Compraison 4..- 50 990 B A Contrast 20 1, 600 C D *Contrast shows the test results conducted on the anodized coating which was not sealed.

EXAMPLE 2 The anodized coating the same as in Example 1 was sealed at 45 C. for 10 minutes using the following composition:

Sodium pyrophosphate-5 g./l. (phosphate ion conc. 2.2

Barium chloride3 g./l. (barium ion conc. 1.97 g./l.)

Tartaric acid0.2 g./l.

Water-Amount necessary for making one liter solution The same tests as in Example 1 were performed on the resultant sealed coating and the results thereof are shown in Table 2, in which the results on the coating sealed by the following methods are shown for comparison.

Comparison 5 The anodized coating the same as in Example 1 was sealed in the following composition at 45 C. for 10 minutes.

Sodium pyrophosphate5 g./l. (phosphate ion conc. 2.2

g./l.) Tartaric acid0.2 g./l. WaterAmount necessary for making one liter solution pH-3.2

Comparison 6 Barium chloride3 g./l. (barium ion conc. 1.97 g./l.)

Tartaric acid0.2 g./l.

WaterAmount necessary for making one liter solution pH--3.2

TABLE 2 Corrosion Abrasion Finger resistance resistance Spot mark Sealing method (sec.) (sec.) dyeing test Number:

6 Example 1 60 1, B A 7 Comparison 5..- 20 1, 600 C C 8 Comparison 6. 2O 1, 600 C D EXAMPLE 3 The anodized coating the same as in Example 1 was sealed in the following composition at 45 C. for-10 minutes:

Sodium phosphate-5 g./l. (phosphate ion conc. 1.34

Strontium chloride3 g./l. (strontium ion conc. 1.65

Potassium permanganate-2 g./l.

Tartaric acid2.6 g./l.

Water'Amount necessary for making one liter solution The same tests as in Example 1 were performed on the resultant sealed coating and the results thereof are shown in Table 3, in which the results on the coating .sealed by the following methods are shown for comparison.

Comparison 7 The anodized coating the same as in Example 1 was sealed in the following composition at 45 C. for 10 minutes:

Sodium phosphate-5 g./l. (phosphate ion conc. 2.9

Tartaric acid2.5 g./l.

Water-Amount necessary for making one liter solution Comparison 8 The anodized coating the same as in Example 1 was sealed in the following composition at 45 C. for 10 minutes:

Strontium chloride3 g./l. (strontium ion conc. 1.65

g./l.) Tartaric acid-0.3 g./l. Water-Amount necessary for making one liter solution pH3.2

Comparison 9 The anodized coating the same as in Example 1 was sealed in the following composition at 45 C. for 10 minutes:

Potassium permanganate-5 g./l. Tartaric acid-0.1 g./1.

WaterAmount necessary for making one liter solution pH-3.2

TABLE 3 Corrosion Abrasion Finger 4 resistance resistance Spot mark Sealing method (seo.) (sec.) dyeing test Number:

9 Example3 65 1,200 B A 10. Comparison 7-.- 20 1,600 O C 11- Comparison 8... 20 1,600 O D Comparison 9-.. 20 1,600 O D EXAMPLE 4 The following two types of sealing composition were prepared, in which the amount and kind of pH adjustor used were varied in accordance with the predetermined 'pH of the composition.

Composition A Disodium hydrogenphosphate-5 g./l. (phosphate ion conc. 1.33 g;/l.)

Calcium chloride.3 g./1. (calcium ion conc. 1.1 g./1.)

pH adjustorA-mount necessary for the pH value shown in Table 4 WaterAmount necessary for making one liter solution Composition B TABLE 4 Corrosion Abrasion Finge Comporesistance resistance Spot markr sition pH (see) (see) dyeing test Number:

13 A 3 8D 1, 200 B A 3 60 1, 150 B A 4 6D 1, 200 B A 4 70 1, 150 B A 6 40 1, 050 B A 6 45 1, 150 B A 8 80 950 B B 8 85 1, 020 B B 10 90 850 C D 10 90 900 C C EXAMPLES 5 TO 24 The following sealing compositions were prepared.

Composition of Example 5 Phosphorous acid-3 g./l. (phosphite ion conc. 2.9 g./l.) Calcium chloride2 g./l. (calcium ion conc. 0.73 g./l.) Rochelle salt-8 g./l.

Water-Amount necessary for making one liter solution pH-3.2'

Composition of Example 6 Sodium phosphite-5 g./(phosphite ion conc. 1.82 g./l.) Strontium chloride3 g./l. (strontium ion conc. 0.98

g./ 1.) 62% HNO 7.6 g./l. Water-Amount necessary for making one liter solution pH3.0

Composition of Example 7 Calcium hypophosphite-IO g./l. (phosphite ion conc.

6.2 g./l. Calcium ion conc. 3.8 g./l.)

Calcium nitrate-5 g./l. (calcium ion conc. 2.62 g./l.)

Rochelle salt5 g./l.

8 Water-Amount necessary for making one liter solution pH3.1

Composition of Example 8 Potassium hypophosphitel5 g./l. (hypophosphite ion conc. 6.8 g./l.) Magnesium nitratel0 g./l. (magnesium ion conc. 2.8

m/l.) 36 HCll.2 g./1. Water-Amount necessary for making one liter solution p-H-3.1

Composition of Example 9 -Sodium pyrophosphate20 g./l.

conc. 7.8 g./l.) Barium acetate-20 g./l. (barium ion conc. 8.7 g./l.) 36% HC19 g./l. WaterAmount necessary for making one liter solution pH-3.2

(pyrophosphate ion Composition of Example '10 Phosphoric acid-30 g./l. (phosphate ion conc. 28.9 g./l.) Calcium acetate-30 g./l. (Calcium ion conc. 8.2 -g./l.) Rochelle salt-100 g./l. Water-Amount necessary for making oneliter solution pH3.l

Composition of Example 11 Barium phosphate-40 g./l. (phosphate ion conc. 12.6

g./l. Barium ion conc. 27.4 g./l.) Strontium carbonate4 g./l. (strontium conc. 1.62 g./l.) Conc. HCl4 g./l. Water-Amount necessary for making one liter solution pH-3.2

Composition of Example 12 Calcium hydrogenphosphate-SO g./l. (phosphate ion conc. 27.6 'g./l. Calcium ion conc. 11.62 g./l.) Barium carbonate5 g./l. (barium ion conc. 1.52 g./l.) 98% H SO -l1.7 g./l. Water-Amount necessary for making one liter solution pH-3.0

Composition of Example 13 Sodium phosphate-3 g./l. (phosphate ion conc. 1.74

g./l.) Magnesium chloride30 g./l. (magnesium ion conc. 7.66

g./l.) Tartaric acid-0.2 g./l. Water-Amount necessary for making one liter solution pH3.3

Composition of Example 14 Sodium tripolyphosphate-30 g./l. (phosphate ion conc.

12.5 g./l.) Strontium ch1oride5 g./l. (strontium ion conc. 1.63

g./l.) 62% HNO 6.2 g./l. Water-Amount necessary for making one liter solution pH-3.2

Composition of Example 15 Phosphorous acid-50 g./l. (phosphite ion conc. 48.3

g./l.) Calcium chloride3 g./l. (calcium ion conc. 1.09 g./l.) Cupric sulfateg./l. Sodium hydroxide-21.5 g./l. WaterAmount necessary for making one liter solution pH-'3.2

Composition of Example 16 Sodium phosphite40 g./l. (phosphite ion conc. 14.56

g./1.) Strontium chloride-5 g./l. (strontium ion conc. 1.63

g./l.) Titan trichloride40" g./l. Sodium carbonate-40 g./l. Water-Amount necessary for making one liter solution pH3.3

Composition of Example 17 Calcium hypophosphite-30 g./l. (hypophosphite ion conc. 22.2 g./l. Calcium ion cone. 7 g./l.)

Calcium nitrate-10 g./l. (calcium ion conc. 5.25 g./l.)

Stannous chloride-50 g./l.

Sodium carbonate2 g./l.

WaterAmount necessary for making one liter solution Composition of Example 18 Potassium hypophosphite20 g./l. (hypophosphite ion conc. 12.1 g./l.)

Magnesium nitrate20 g./l. (magnesium ion conc. 9.67

Ammonium vanadate-3 g./1.

Rochelle salt-1.1 g./l.

WaterAmount necessary for making one liter solution Composition of Example 19 Sodium pyrophosphatel g./l.

conc. 7.7 g./l.)

Barium acetate-30 g./l. (barium ion conc. 13.1 g./l.)

Antimony trichloride g./l.

WaterAmount necessary for making one liter solution (pyrophosphite ion Composition of Example 20 Phosphoric acid-5 g./l. (phosphate ion conc. 4.8 g./1.) Calcium acetate40 g./l. (calcium ion conc. 17.4 g./l.) Potassium dichromate g./1. Rochelle salt20 g./l. WaterAmount necessary for making one liter solution pH3.0

Composition of Example 21 Calcium hydrogenphosphate--50 g./l. (phosphate ion conc. 19.2 g./ l. calcium ion conc. 7.93 g./l.) Barium carbonate-3 g./l. (barium ion conc. 0.91 g./l.) Potassium permanganate3 g./l. 98% H SO 7.3 g/l. WaterAmount necessary for making one liter solution pH3.2

Composition of Example 23 Sodium phosphate-3 g./l. (phosphate ion conc. 1.74

Magnesium chloride50 g./l. (magnesium ion conc. 12.7

Iron alum3 g./l.

Rochelle salt16 g./1.

Water-Amount necessary for making one liter solution Composition of Example 24 Sodium tripolyphosphate3 g./l. (phosphate ion conc.

Strontium ch1oride3 g./l. (strontium ion conc. 0.974

Formalin50 g./l.

62% HNO --4.8 g./l.

WaterAmount necessary for making one liter solution The anodized coating the same as in Example 1 was sealed at C. for 10 minutes using the above compositions of Examples 5 to 24 and the same tests as in Example 1 were performed in the same manner as in Example 1 with the results shown in Table 5 below.

TABLE 5 Corrosion Abrasion Finger resistance resistance Spot mark Composition (see.) (5%.) dyeing test Number:

23 Example 5 60 1,100 B A 24 Example 6. 1, 250 B A 25...- Example 7...- 55 1,100 B A Example 8. 1, 100 B A Example 9. 60 1,150 B A Example 10- 60 1,150 B A Example 11 60 1, 150 B A Example 12- 60 1, 200 B A Example 13 60 1, 100 B A Example 14- 60 1, 050 B A Example 15 1, 200 B A Example 16 65 1, 200 B A Example 17 1, 200 B A Example 18 65 1, 150 B A Example 19 70 1, 200 B A Example 20 70 1, 150 B A Example 21.-- 70 1,200 B A 40 Example 22- 65 1, 200 B A 41 Example 23 70 1,150 B A 42 Example 24- 70 1, 200 B A EXAMPLE 25 The following composition was prepared:

Sodium phosphite10 g./l. (phosphite ion conc. 3.65

Magnesium chloride-3 g./l. (magnesium ion conc. 0.35

Tartaric acid5 g./l.

WaterAmount necessary for making one liter solution The anodized coating the same as in Example 1 was sealed at temperature shown in Table 6 below for 10 minutes using the above composition and the tests performed in the same manner as in Example 1 gave the following results.

TABLE 6 Corrosion Abrasion Finger Temperaresistance resistance Spot mark ture C.) (see) (sea) dyeing test;

20 50 1, 150 B A 30 55 1, B A 40 60 1, B A 50 60 1, 200 B A 60 60 1, 200 B A 70 60 1, 200 B A What is claimed is:

1. A method for sealing aluminum oxide coating anodically formed on aluminum and aluminum base alloy, which comprises contacting said coating with a composition consisting essentially of an aqueous solution containing at least one g./ liter of oxygen acid ion of phosphorus and at least 0.2 g./ liter of alkaline earth metal ion and having a pH of 2.5 to 9.0 at 20 to 50 C., and drying the resultant coating.

References Cited UNITED STATES PATENTS 2,528,787 11/1950 Roland 148-6.15 R 3,015,593 1/1962 Jayne 1486.l5 R

RALPH S. KENDALL, Primary Examiner US. Cl. X.R. 

